The detailed structure of bark varies greatly in different trees, though its general principles remain unaltered. One of the best short accounts of these is given by Prof. H. Marshall Ward on page 199 of his little book on ‘Timber and some of its Diseases,’[133] and further information may be found in Van Tieghem’s ‘Traité de Botanique’ and other works on structural botany. [133] Macmillan & Co. With regard to detailed structure of various tanning barks, ‘Die Gerberinden’ of von Höhnel[134] is one of the best authorities. [134] ‘Die Gerberinden,’ Berlin, 1880. The inner surface bark of a young tree, or twig, consists of a layer of soft and living cells resting on the outer surface of the wood, and called the _cambium_. These cells multiply by division (cp. p. 12) and produce from their inner surface the successive annual layers of wood, and on their outer a fibrous tissue called the bast (_phloem_), consisting of lengthened cells, and tubes with perforated divisions (sieve-tubes) which convey sap, and mostly run in the direction of the branch, but are crossed transversely by cells in a line with the medullary rays of the wood. All these cells when first produced in the cambium-layer have thin and soft cellulose walls, but the inner layer forming the wood becomes lignified, or hardened, by deposits of lignine on the interior of the cell-walls, while their contents of living protoplasm disappear. The outer layer forming bast remains much softer and more fibrous, and retains its vitality longer. The outer surface of the young branch is covered by a thin layer of flat cork-like cells forming the epidermis, developed from the growing tissue of the bud, beneath which is a layer of growing cells frequently called the _cork-cambium_. This produces, on its inner side, a layer of soft, juicy, thin-walled cells (_parenchym_), which are living and capable of growth, and contain protoplasm and often chlorophyll, to which the green colour of young twigs is due. This layer at first rests on the bast. On the outer side, the cork-cambium produces corky layers beneath the epidermis. The section of an oak-twig is shown in Fig. 43. [Illustration: FIG. 43.--Section of Oak Twig, drawn by Prof. Bastin: _c_, corky layer; _t_, tannin-cells; _St_, stone-cells; _Ca_, cambium; _Mr_, medullary ray; _P_, pith.]